This invention relates to a digital symbol generator with symbol error checking for developing analog deflection voltages to drive a cathode ray tube display, forming a plurality of symbols from successive blanked and unblanked vectors rotatable about an arbitrary angle of rotation.
Digital symbol generators have been known in which the vectors forming the symbol may be rotated. The data specifying individual vectors has consisted of either polar coordinates, angle and length information or cartesian coordinates, starting and end point information. Where polar coordinates have been used, the angle destination has been a fixed binary code resulting in a limited number of angles at which the vectors can be drawn or about which the vectors may be rotated. Where cartesian coordinates have been used, rotation of a vector has been accomplished either by complex digital transformation of the cartesian coordinates or analog devices which rotate the entire symbology at the input to the display.
A computer typically supplies a digital symbol generator with the input data required to specify the length and orientation of the vectors forming a symbol. Where the computer supplies both starting point and end point data for each vector, the symbol generator is highly dependent upon the computer for input information. Other symbol generators include a memory for storing vector data. The memory is then incrementally addressed to sequentially read out the data required for each vector forming a symbol. This type of symbol generator is less dependent upon the computer, however, when changes in the symbology are required the memory must be reprogrammed which is very costly and time consuming.
Digital symbol generators further utilize constant speed stroke writing techniques to ensure uniform brightness of the displayed symbols. One such technique is to normalize the x and y axis displacements by means of shift registers. Since all vectors generated by this method are drawn at the same speed, effective drawing time is wasted where the vectors are not visible on the display.
One of the many symbol generator uses is in an aircraft to display flight guidance information on a pilot display unit. This requires extremely accurate symbology to ensure the safe operation of the aircraft. Symbol error checking has been accomplished in the past by analog error rate processors which are typically very costly.